1. Technical Field
The invention concerns a cranio-cerebral protective helmet adapted to the anatomy of the head and to the neurosurgical knowledge.
2. Background Art
The cranium comprises two portions: the neuro-cranium which contains the brain and the viscero-cranium which represents the skeleton of the face. The present invention mainly concerns the part of the helmet covering the neuro-cranium.
Protective helmets have:
A. two components which must answer to biomechanical safety requirements:
1. an external shellxe2x80x94hereafter calledxe2x80x9cthe shellxe2x80x9dxe2x80x94which ensures, during an impact, the distribution of the delivered energy to a surface larger than the surface interested by the external shock. It also ensures an increased resistance to the penetration of the helmet and the sliding of the helmet on different surfaces in case of accident;
2. an intermediate capxe2x80x94hereafter calledxe2x80x9cthe capxe2x80x9dxe2x80x94intended for the energy absorption by its compressive crush during an impact;
B. an internal component also called comfort stuffing, dedicated to improve the user""s comfort.
Certain helmets also have intermediate shells. The termxe2x80x9cshellxe2x80x9d as used in this description corresponds to the external shell as well as to any other intermediate shell.
In the case of the helmets for motorcyclists"" (or similar) application, by the use of increasingly resistant materials, the shell distributes on the surface and transmits to the intermediate cap almost all the received energy, even in the case of an impact with high neurological risk (INR). In the case of the helmets for cyclists (or similar), by the use of very flexible/soft materials, the shell does not ensure, or practically not, any biomechanical function. In all the cases, after the partial absorption of energy by the intermediate cap, the residual kinetic energy is transmitted to the cranium and finally to the brain. The immediate neurological disorders which may result are more serious if the energy transmitted to the brain is higher. By xe2x80x9cINRxe2x80x9d is meant impacts which expose the subject to neurological disorders (transitory or persistent) in spite of the usage of a helmet designed in the state of the art.
The conception of the current helmets encounters several problems:
How to increase their effectiveness without increasing their thickness and their volume beyond the acceptable limits? Beside the lack of comfort and the tiredness of the neck muscles, the volume increase may cause accidents itself by the reduction of the surrounding visual and hearing perception. In the mean time, the exaggerated increase of the helmets"" volume and/or weight can easily slow down their usage.
When to protect the head better: in violent impacts (rare) or in moderate impacts (frequent)? Corner (1987), Mills (1991), Smith (1993) showed that if the current helmets are designed for better deadening the violent impacts, they will be hard and less effective in the event of a mild energy impact.
Another disadvantage of the current helmets is related to the fact that the hardness of their cap is not adapted to the resistance of the various areas of the cranium. In particular, the resistance of the cranium varies a lot from one area to another because of the differences in thickness of the cranium (less than 2 millimeters in the anterior temporal area, almost 10 millimeters in the parietal area); the various curvature radii of the skull vault; and the presence of the cranial sutures.
The invention provides for the reduction of the cranio-cerebral lesions and the post traumatic neurological disorders by an important energy absorption in the event of a violent impact, through the deformation or the fracture of the helmet""s shell in front of the zones of maximum resistance of the cranium and by a better protection of the cranium due to an intermediate cap having a variable hardness or density adapted to resistance of the various areas of the skull vault.
Both the shell and the cap of the helmet herein described contribute to solve these problems.
The shell of the helmet herein described has the capacity to undergo deformations or fractures preferably in front of the areas of maximum resistance of human cranium, in the event of INR. The energy thus absorbed or consumed ensures the reduction of the energy transferred to the head and to the cervical spine also. The risks of post-traumatic tetraplegia secondary to a fracture of the cervical spine will thus also decrease. The deformations or the fractures occur in areas of the shell containing low mechanical resistance layers (LRL) or low mechanical resistance zones (LRZ). In a low energy impact, the shell according to the invention functions on the same principle as the shell of the current motorcycle helmets. From this point of view the LRZ and the LRL act as thexe2x80x9crelief valvesxe2x80x9d of the pressure containers.
For obtaining a differentiated pressure repartition on the fragile and resistant zones of the cranium during an impact, the cap of the helmet herein described has a density or a hardness variable and adapted to the resistance of the different areas of the skull vault. Thus the helmet herein described comprises a cap with zones of low compressive-crushing resistancexe2x80x94softxe2x80x94in front of the fragile zones of the human cranium, and zones of high compressive-crushing resistancexe2x80x94hardxe2x80x94in front of the zones of maximum resistance of the human cranium. Through this logical distribution of the hard and soft zones of the cap, one obtains a more effective helmet in comparison with ones currently available, but having a similar volume and weight.
The deformation or the fracture of the shell has important biomechanical consequences:
1. the (t) duration of the impact increases.
2. the kinetic energy (EC=mV2/2) received by the head (Ec3) decreases since the energy absorbed by the helmet (xcex94E1+xcex94E2) increases.
Ec1=kinetic energy of the ensemble before the impact
xcex94E1=energy absorbed by the shell
xcex94E2=energy absorbed by the cap
Ec3=Ec1xe2x88x92(xcex94E1+xcex94E2)
The average acceleration (a) decreases because Ec3 decreases and T increases.
(a=V/t=(2xc3x97Ec3/m)1/2 /t).
The Head Injury Criterion (HIC), used to evaluate the damping of the normative impacts, is expressed in its simplified form:
HIC=dV2.5/dt1.5=(dV2/dt)(dV/dt)1/2 
It is proportional to the kinetic energy (dV2) and inversely proportional to the duration of the energy transfer during the impact (dt). For the reasons already exposed, it will decrease; thus presenting a better damping of the shocks.
The embodiments presented below are only given as examples and are not restrictive. Various combinations between the different embodiments disclosed and their alternatives are also considered.